In the design and construction of transducers employing the elastic response of a structure, a frequent goal is the isolation of a small, high-strain region that is particularly sensitive to a single component of interest. Such components might include components of load, moment or acceleration. Typically this region is instrumented with a sensor such as an electrical strain gauge. A motivation for placing the gauge in a high-strain region is that the maximum strain seen by the gauge is a limiting factor in its sensitivity. Unfortunately, incorporating high-strain regions in a device usually precludes its use as a direct or critical load bearing element. One consequence of limiting the strain in a structure is that "smart" devices (e.g. "smart structures") incorporating imbedded strain sensors (e.g. piezo-electrics) would have commensurately lower sensitivity.
An alternative to using strain sensors whose sensitivity scales with the maximum strain in the instrument is to use a displacement sensor whose sensitivity scales with maximum displacement. A displacement sensor yields a strain measurement by dividing the displacement by the length of the sensor. Thus, if the strains of interest are limited in magnitude, a displacement sensor of sufficient length could be used in lieu of a strain sensor. A device using displacement sensors may tend to require longer measurement paths than an equivalently sensitive instrument employing strain sensors. Typically components of interest are extricated through a balanced bridge of sensors. One difficulty in increasing the measurement length, especially in a multi-component instrument, is that the separation of these components is usually based on an approximation of constant strain over a small region. Designing a transducer based on this approximation that utilizes displacement sensors and increases the measurement length would tend to exacerbate the cross-talk between the component measurements. There are numerous methods to measure the properties of a deforming body. A typical laser fiber optic interferometric strain gauge is taught by Butter in U.S. Pat. No. 4,191,470. Moire interferometers have also been used to determine structure deformation, as can be seen in Dason, U.S. Pat. No. 4,850,693. None of the prior art, however, successfully addresses the limitations above discussed. The invention disclosed overcomes these limitations.
Load transducers are described that, within the confines of elastostatics, have zero cross-talk amongst the resultant component measurements. A six-component load transducer can be constructed using either electrical or optical sensors of finite measurement path length. The electrical strain gauges can have zigzagging paths and still retain the component separation. The Mach-Zehnder optical sensors allow an instrument in which the signal (i.e. total phase change) at maximum load grows with the geometry. For a fixed maximum load and signal measurement capability this geometric scaling corresponds to an increase in the precision of the load measurement.
An improved torsion transducer is also disclosed in which the precision of the device scales with the number of paths, while the number connections to the instrument remains constant. From this design, a "solid" torsion transducer is presented for which the fiber optic paths are the entire load bearing portion of the instrument. In a further embodiment, a linear acceleration sensor and a structural integrity sensor that is invariant to all end loading are disclosed.
An important consequence of the fiber-optic geometric scaling is that the optical instruments can have lower material design strains than electrical transducers. This lower strain allows the actual instrument to be a direct load bearing structure. Another consequence of this geometric scaling is that the load instrument need not be constructed of structurally efficient materials. For instance, for a constant load and design strain, a lower modulus construction material would result in a bigger geometry. This larger geometry corresponds to a longer fiber-optic sensor path and thus a larger signal.